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Drilling down into Dunedin’s floods

Young Dunedinites have been boring holes to understand why their city is so waterlogged. Will their findings help locals prevent or prepare for future flooding?

Year 10 students at King’s and Bayfield High School in South Dunedin have been exploring the geology of groundwater and soil type in their flood-prone school grounds in a new project to find out what’s beneath their feet.

Project lead Dan Hendra, who is also Director of the New Zealand International Science Festival, explains that the project seeks to shed light on the origins of 2015’s extreme flooding event.

“This was a major crisis in the area that affected both of the schools, so learning about what caused it is immediately relevant to the students,” he says.

The students began by boring several holes in the ground, with help from experts from GNS Science and Otago Regional Council’s Natural Hazards team. They attached a sensor to one of the boreholes, which would help them monitor changes in the water temperature and conductivity (concentration of salt and other dissolved minerals) in the ground.

With the other holes, the pupils didn’t attach sensors but instead analysed the cores – the cylinders of soil taken from the ground. They used these to find out about the different layers of the soil and how far down the water table begins (the level where the ground is water-saturated).

“The boys have been really into this and are now putting together the pieces to understand the geological characteristics of this area, ” says Simon Cushen, head of Social Science at the all-boys King’s High School.

“I really enjoyed looking at the cores up close and seeing how the soil changed from being grassy and dry at the top to wet mud and clay at the bottom," says 15-year-old King's student Lewis Wall. "It was really interesting seeing where the water table began.”

Classmate Samuel Standring, 14, adds, “My favourite part was seeing the scientists’ diagrams and hearing his explanations about what we’d found in the cores.”

Jake Remon, 14, says, “I liked digging the hole and finding the glass piece quite far down the core, because that shows people have been living here a while.”

In another session, the students recreated an aquifer (area of waterlogged rock) using plastic cups filled with gravel, sand and clay to see how water moves through the ground. They found that the water flowed through the gravel fastest but hardly moved through the clay.

Ellyse Gore at Otago Regional Council explains that clay has much less ‘pore space’ than gravel, due to having smaller gaps between its finer grains, so water takes longer to pass through. She says this experiment showed the students how ground variation can influence whether water ponds on the surface or not, and how long this ponding takes to go away.

“When it comes to South Dunedin,” she says, “the sediment is made up of a variety of sands, silts and clays, which respectively have decreasing pore space.”

Many of South Dunedin’s houses are also built on reclaimed land – in which boggy wetlands and dunes are artificially filled in to make them more solid – but that ground is still less than a metre above sea level.

Simon Cox, Principal Scientist at GNS Science, says this is particularly true for the school grounds: “We think the school grounds were originally a lagoon or similar body of water that was filled in to make a park or something before the school was built here.”

Dan adds, “I thought it was quite scary finding out that the groundwater under King’s High School is only 60 centimetres below the surface!”

Once the students had collected and analysed all their data, they shared the results with locals. Ellyse says that the students’ findings confirm that South Dunedin is particularly flood-prone due to being a low-lying settlement close to the sea, plus having unfavourable soil types, a shallow water table and high levels of rainfall.

The sensors are still connected to the school boreholes and the data from these continue to be monitored by the Otago Regional Council as well as the school students, to paint a clearer picture of any long-term trends in water changes.

About the project

What Lies Beneath is run by the New Zealand International Science Festival in partnership with GNS Science, Otago Regional Council and the University of Otago, with support from the Unlocking Curious Minds contestable fund.